1. Field of the Invention
The present invention relates to refrigerating machine oil compositions, and more particularly to refrigerating machine oil compositions having a low viscosity, an improved energy saving performance, a good sealability and an excellent compatibility with a refrigerant which are suitably used in various refrigerating application fields, in particular, closed-system refrigerators.
2. Description of Related Arts
In general, compression-type refrigerators are constituted from at least a compressor, a condenser, an expansion mechanism (such as expansion valve), and an evaporator and/or a dryer, and have such a structure in which a mixed liquid composed of a refrigerant and a lubricant (refrigerating machine oil) is circulated through the closed system. In the compression-type refrigerators, in general, since an inside of the compressor is kept at a high temperature whereas an inside of the cooler is kept at a low temperature although these conditions vary depending upon kinds of refrigerators, it is required that the mixed liquid is circulated through the system without undergoing a phase separation thereof into the refrigerant and the lubricant over a wide temperature range of from the low temperature to the high temperature. The refrigerant and the lubricant in the mixed liquid generally have phase separation ranges on high-temperature and low-temperature sides, respectively. A maximum temperature in the low-temperature side phase separation range is preferably −10° C. or lower and more preferably −20° C. or lower whereas a minimum temperature in the high-temperature side phase separation range is preferably 30° C. or higher and more preferably 40° C. or higher. If the mixed liquid undergoes the phase separation during operation of the refrigerators, the life or operation efficiency of these apparatuses tends to be considerably adversely affected. For example, if the mixed liquid suffers from phase separation into the refrigerant and the lubricant in the compressor, poor lubrication occurs at a movable part thereof, resulting in defects such as seizing and, therefore, remarkably shortened life of the apparatuses. Whereas, if the phase separation occurs in the evaporator, a heat exchange efficiency of the apparatuses tends to be deteriorated owing to a high-viscous lubricant being present therein
Hitherto, as the refrigerant for refrigerators, there have been mainly used chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), etc. However, these compounds contain chlorine causing environmental problems. For this reason, there have been presently made studies for developing alternate refrigerants containing no chlorine such as hydrofluorocarbon (HFC). However, HFC has been reported to have adverse influence on global warming. Under these circumstances, there have been noticed so-called natural refrigerants suitable for environmental protection such as hydrocarbons, ammonia and carbon dioxide.
In addition, since the lubricant for refrigerators are used in order to lubricate movable parts thereof, it is also important that the lubricant has a good lubricating performance. In particular, since an inside of the compressor is likely to be kept at a high temperature, it is important that the lubricant has a viscosity capable of retaining an oil film required for a good lubrication. Although a necessary viscosity of the lubricant varies depending upon kinds of the compressor and use conditions thereof, the lubricant before mixing with the refrigerant preferably has a viscosity (kinematic viscosity) of 10 to 200 mm2/s as measured at 40° C. It has been conventionally reported that if the viscosity of the lubricant is lower than the above specified range, a thickness of an oil film formed tends to be reduced, resulting in poor lubrication, whereas if the viscosity of the lubricant is higher than the above specified range, a heat exchange efficiency of the refrigerators tends to be lowered.
For example, there is disclosed a lubricant composition for vapor compression-type refrigerators using carbon dioxide as a refrigerant which comprises a lubricant base oil having a 10% distillation point of 400° C. or higher and a 80% distillation point of 600° C. or lower as measured by a gas chromatograph distillation method, a kinematic viscosity of 2 to 30 mm2/s as measured at 100° C., and a viscosity index of 100 or more (e.g., refer to Japanese Patent Application Laid-open No. 294886/2001).
In Examples of the above Japanese Patent Application, it is described that the kinematic viscosity of the base oil used in the lubricant composition is in the range of from 17 to 70 mm2/s as measured at 40° C.
The refrigerators using a refrigerating machine oil having such a high viscosity tend to inevitably suffer from a large energy consumption. Therefore, for the purpose of saving energy for the refrigerators, studies have been made to reduce a viscosity of the refrigerating machine oil and improve frictional properties upon the lubrication.
For example, in refrigerators for cold storage, an energy saving performance thereof has been improved by reducing a viscosity of the refrigerating machine oil used therein to VG32, 22, 15 and 10, respectively. However, when the viscosity of the refrigerating machine oil is further reduced, there tends to arise such a problem that a sealability and a lubricating property thereof are deteriorated.